The present invention relates to pellicles that are used in the semi-conductor chip industry, and more particularly to a method and apparatus for manufacturing pellicles.
In the semi-conductor chip industry it is well known that pattern transfer from a photo mask to a substrate is accomplished by exposing the photo mask to a light source. During the pattern transfer process, also called the photolithographic process, circuit patterns on the photo mask are projected onto the substrate which has been treated with a photo-sensitive substance. This results in the patterns being reproduced on to the substrate. Unfortunately, any foreign substance on the surface of the mask will also be reproduced on the substrate and therefore will interfere with proper pattern transfer.
To eliminate contamination of the mask surface, a framed, thin membrane known as a pellicle is mounted on it, such that the pellicle membrane extends parallel to the mask, at a predetermined distance spaced from the mask. Any contaminates which would ordinarily land on the mask surface fall onto the pellicle membrane, and not the mask. Because the frame of the pellicle supports the membrane at a spaced distance from the mask surface, particles or other contaminants on the pellicle membrane will be out of focus during pattern transfer, and therefore will not be projected onto the substrate. The use of pellicles in manufacturing integrated circuits can increase the quality of the resulting circuits, thereby dramatically improving circuit fabrication productivity.
Because high quality pellicles are critical to the success of the photolithographic process, pellicle manufacturing techniques have become increasingly important. Specifically, during the pellicle manufacturing process, it is important to minimize the possibility of contaminant particles being deposited on the pellicle membrane.
In the event of a relatively large particle being deposited on the pellicle membrane, such particle may be reproduced on the substrate during photolithography even though such particle may be out of focus. Additionally, if a contaminant particle temporarily attaches to the underside of the pellicle membrane or the pellicle frame, such particle may drop onto the mask surface and contaminate it during photolithography. This is precisely what is to be avoided by using a pellicle.
It is also critical that the pellicle membrane be extremely uniform so that light rays passing through it during photolithography are unobstructed and are not in any wa refracted or otherwise bent. The composition of the membrane must be highly uniform, and the membrane must be evenly tensioned across the pellicle frame. Also, it is important to ensure that a continuous seal exists between the thin film and the frame.
To further understand these important requirements, it is necessary to provide an explanation as to how pellicles are formed.
As is known in the art, forming an optical membrane is the first step in pellicle manufacture. Commonly the membrane is prepared by spinning a suitable polymer, such as nitrocellulose or a nitrocellulose-containing polymer, on a substrate. The newly formed membrane is then removed from the substrate and held, adjacent its periphery under tension, to prepare it for subsequent manufacturing steps.
Next, a frame is fastened, or bonded, to a working area of the membrane, framing the working area. After fastening, the frame and the attached working area of the membrane are ready to be separated from the remainder of the membrane. It is this separating step that is of the utmost importance to high quality pellicle manufacture. For it is during the separating step that the framed working area must be cut away from the remainder of the membrane. Using the method and apparatus of the present invention a surprisingly successful separation is obtained.
Currently, it is known to remove the framed working area by cutting the membrane outward of the frame using a suitable knife, or razor blade. The problem with such a procedure is that the initial puncturing of the membrane by the knife causes shattering of the non-working area which produces contaminant particles that may collect on the frame or the working area.
Additionally, the membrane may tear in undesired directions. If the tear continues through the bond between the frame and the membrane and into the working area, the pellicle must be rejected because it will not perform its function when the integrity of the working area is damaged.
An even greater problem exists when the working area has been microscopically distorted due to a variation in the tension across it. This type of distortion, which may not be visible to the naked eye(s) of quality control personnel, can affect the uniformity of the membrane. Because it is essential that the pellicle membrane transmit light uniformly without refraction/bending, this distortion can have catastrophic consequences. While equipment can be, and often is, used to detect such problems in the membrane, such equipment is expensive and its use can slow down the production process.
Accordingly, it is a general object of the present invention to provide a unique, shatter-free, tear-free apparatus and method for controllably separating or removing a framed working area of a membrane from the remainder of the membrane.